What difference is there between そろそろ and もうすぐ? (if any)

Question

そろそろ and もうすぐ can both be translated as 'soon', but I feel they have different uses and connotations.

My take on it is that そろそろ is slightly more formal and has more of a meaning of 'It's about time (we)...' as in this example:

そろそろ出ましょうか？ Shall we get going? / Isn't it about time we go?

Whereas もうすぐ sounds more urgent, and has more of a meaning of 'almost' or 'just about to', like in the following example:

もうすぐ夏休みだ。 Summer vacation is just around the corner.

Can these be used interchangeably? Can I say the following sentence:

そろそろ夏休みだ。

To me it sounds a bit stiff and formal.

Answer 1

I see little difference in the level of formality. もうすぐ may be, relatively, a bit more casual or colloquial than そろそろ, but, I can't say that そろそろ is a formal or stiff word in general.

Only そろそろ has the meaning of "expected time", "high time", "it's about time". You can just say 「そろそろ…。」 when you want to leave now, to interrupt a boring discussion, or to change a situation in general. You cannot say 「もうすぐ…。」 in such cases.

And in your example:

もうすぐ夏休みだ。
そろそろ夏休みだ。

Both of these are okay, and the difference is small. But if you're innocently celebrating the upcoming, happy vacation, I think the former fits better. To me, the latter might sound a bit less exciting, or might sound like some kind of action is expected.

もうすぐ夏休みだ！ いっぱい遊ぶぞ！
もうすぐクリスマスだから、サンタさんに手紙を書いたよ！
もうすぐお正月！ (in an ad for a travel package, etc.)

そろそろ夏休みだ。旅行の準備をしないと。
そろそろ夏休みが終わっちゃう。宿題が残ってるのに…。
そろそろクリスマスだけど、どう過ごすのか予定決まった？
そろそろお正月！ (in an ad for year-end oosoji goods, etc.)